Rather than an allied race, which won’t happen now, because they’re are just the same as Blood Elves - I think this idea, from MMO champion, is actually the best we can hope for, for all races.
Why go through all that when you could just have racial skins?
Just have racial skins - there’s no way the highborne would be neutral or friendly towards the Horde, considering it was the Horde that culled them twice.
Racial skins for Night Elves could be:
Regular Night Elf
Highborne Night Elf (Mage, Warlock)
Darkfallen Night Elf (Hunter, Rogue, Death Knight)
Illidari Night Elf (Demon Hunter, Warlock)
Then you just pick which one you’d like. It’s not directly impactful on the lore, but gives players more options.
Humans could be:
Stormwindian Human
Lordaeron Human
Arathor Human
Blood Elves could be:
Regular Blood Elf
Sunfury Blood Elf (Mage, Warlock)
Illidari Blood Elf (Demon Hunter, Warlock, Mage)
Darkfallen Blood Elf (Hunter, Rogue, Warrior)
I don’t think the “class” specific skins work, because it’s trying to pidgeon-whole people into just playing a certain class with a certain race skin.
For Void Elves - rather than going “Farstrider”, just go:
Regular Void Elf
Quel’dorei
Darkfallen
That way - it leaves the door open to either play a Quel’dorei Farstrider, Quel’dorei Magister or Ren’dorei Warlock.